1. Field of the Invention
The present invention relates to a fixing member for an electromagnetic induction heating for use in a device utilizing an electrophotographic system such as a copier, a printer or the like, to a fixing device using an electromagnetic induction heating method, and to an image forming device having the fixing device.
2. Description of the Related Art
In an image forming device such as a copier or a printer employing an electrophotographic system, the process of fixing a toner image formed on a recording material such as paper to make a permanent image has been conventionally called a “fixing process”. Conventional fixing processes include methods of press fixing, oven fixing, and solvent fixing, however, the thermal press fixing method has been most commonly used. This is due to the fact that the thermal press fixing method can effectively transmit heat and fix the toner image more firmly than other methods, and furthermore, it is comparatively safe.
The thermal press fixing method is a method in which a recording material having an unfixed toner image formed thereon is passed through a nip formed by two heated rolls or belts. The unfixed toner, which is heated by the rolls or belts and brought into a fused state when passed through the nip, is pressed onto the recording material and fixed thereto by the nip pressure.
The roll or the belt of a fixing member has a releasing layer provided on its surface, so as to have good separability and to prevent the surface from being fixed to the fused toner. Further, the roll or the belt is heated by a heating member in order to transmit heat to the toner image.
A method of heating the roll or the belt from inside the roll with the radiant heat of a halogen heater, which is provided in the roll, has been conventionally used. With this method, it takes much time to heat the surface of the roll to be heated to the point where the toner image can be fixed, because the roll is heated from the inside. For this reason, when a user copies or prints something, it is necessary to wait for the printed item. Moreover, in order to make the waiting time as short as possible, the surface of the fixing roll is continuously heated at a high temperature during standby so as to maintain a temperature that is lower than the fixing temperature. However, this method increases power consumption due to the standby heating, hence, the method does not satisfy the recent demand to provide energy-efficient machines.
Thus, a fixing device using a thin film and a fixed heater is taught utilizing an energy-saving fixing method in certain patent documents such as Japanese Patent Application Laid-Open (JP-A) Nos. 63-313182 and 4-44074. There has also been a widely used method of using a thin film belt as the fixing member, and heating the fixing member with a planar-resistant heating body arranged in the thin belt. In this method, as compared with the method of heating the roll from within the center, it is possible to shorten the fixing time because the belt can be heated without a heat insulation air layer, and further, the method does not require heating the center of the roll.
However, in the method using the above-mentioned belt and planar-resistant heating body, the planar-resistant heating body itself possesses a heat capacity, and it is difficult to shorten the time necessary to reach a fixing temperature to the point that the user does not feel a waiting time. It is also difficult to make the temperature distribution of the planar-resistant heating body uniform in the axial direction. Therefore, considering the current state of the above-mentioned method, sufficient energy conservation and high-quality image forming have yet to be achieved.
Meanwhile, a method of heating a fixing member with an induction heating system has been studied in recent years (e.g., JP-A Nos. 11-352804, 2000-188177). The heating principle of an electromagnetic induction heat-fixing system will be explained below.
The electromagnetic induction heat-fixing system requires not only a heat-fixing member and a press member, which are conventionally used, but also a coil and a high-frequency power source. The coil is arranged at a position inside the heat-fixing member or outside and near the heat-fixing member, and is electrically connected to the high-frequency power source. A metal heating layer in either the shape of a roll or a belt can be used as the heat-fixing member, which is heated by electromagnetic induction.
A high-frequency alternating current is passed through the coil from the high-frequency power source. At this time, magnetic flux is generated in the coil in a direction perpendicular to a plane wound by the coil corresponding to the direction of the current. The magnetic flux crosses the metal heating layer of the heat-fixing member arranged near the coil, generating an eddy current that in turn generates a magnetic field in a direction canceling this magnetic flux generated in the metal heating layer. Since the resistance of the metal heating layer is determined by the type of metal and the thickness thereof, the electric energy of the generated eddy current is converted to thermal energy. A fixing device using heat generated in this manner is referred to as an electromagnetic induction heat-fixing device.
Since the surface of the fixing member is heated by the heating of the metal heating layer at this time, when the recording medium on which an unfixed toner image has been formed passes through the nip constituted by the fixing member and press member, the unfixed toner image is heated, pressurized, adhered, and fixed. In this method, the surface of the member to be heated can be heated effectively and thermally efficiently, making it possible to shorten the time necessary to reach a fixing temperature to an absolute minimum. As described above, the induction heat-fixing device includes a roll-type device and a belt-type device. In both types, by running a high-frequency current through the coil arranged near the member to be heated, an induced electromotive force is generated in the metal heating layer of the member, creating the eddy current that heats the member. In the roll-type device, a core metal can comprise the heating layer and be heated to a fixing temperature if an appropriate material is selected. The core metal material should be of a thickness capable of generating the eddy current with the coil, and heating the member with the eddy current.
However, in the case of a roll-type device, it is the core metal that is heated, so the fixing temperature can be reached in a shorter time. This is because unlike conventional heating systems, there is no air layer, however, the core metal needs to have a thickness of several millimeters because it must possess rigidity. As a result, the core metal of the heating layer inevitably has a large heat capacity, which in turn increases the time it takes to heat the core metal. Accordingly, it is impossible to sufficiently shorten the time it takes to reach the fixing temperature.
Methods of forming a belt-type induction heat-fixing member include a method of using the metal heating layer as a substrate, and a method of forming a metal heating layer on a heat-resistant resin substrate. In the case of a belt using a metal heating layer as the substrate, the thickness of the substrate of the metal heating layer needs to be dozens μm to 200 μm thick because the substrate needs to be strong to a certain extent. This increases the heat capacity of the substrate, which increases the amount of time necessary to heat the surface of the belt, though not to the same extent as the roll-type device.
Further, in order to form a nip with a press member and the belt, it is necessary to arrange a pressure applying member at a position opposite to the belt inside the belt. In many cases, a rubber pad is used as this pressure applying member because it forms the nip with the press member at a uniform pressure and ensures a nip width, however, this pad does not slide well against the metal substrate and is thus prone to intense deterioration.
Meanwhile, in the case of a belt using a substrate made of heat-resistant resin, engineering plastic having a heat resistance of 200° C. or more and having sufficient strength, such as polyimide or polyamide imide, is used. In this case, because the resin substrate ensures strength, the metal heating layer can be thinned as long as it can generate a sufficient amount of heat. Thus, in comparison with a belt having a metal substrate, it is possible to shorten the time it takes to reach the fixing temperature. Moreover, since the substrate is resin, it slides well against the pad inside the belt forming the nip.
The metal heating layer needs to be formed on the substrate in a uniform thickness. In certain cases, depending on the type of metal, the thickness of the layer can be decreased if the metal has low resistance, hence, it is possible to reduce the time it takes to reach the necessary fixing temperature. Generally speaking, metals such as copper, aluminum, and nickel are often used for the metal in the heating layer.
Using these metals, a thin metal film can be formed on the heat-resistant resin with methods such as plating, vapor disposition, and sputtering. As described above, there is an optimum thickness, depending on the type of metal used, and the thinner the thickness, the less rigid the belt itself becomes. A thinner belt is more flexible, making it easier to form a suitable nip, thereby forming a fixed image of better quality. In addition, the heat capacity of a metal heating layer with a thinner film can be decreased, providing the advantage of shortening the time required to reach the necessary fixing temperature. It is therefore necessary to select a metal that has low resistance and that can heat despite being thin, and to form the metal film as thinly and uniformly as possible.
However, in the current state of art, there are problems such that the durability of the thin metal heating layer is insufficient when the fixing belt in which the above-mentioned thin metal heating layer is formed on the resin substrate is used.
The thinner the film of the metal heating layer is, the less the heat capacity becomes, hence, the time required for the metal heating layer to reach the fixing temperature becomes shorter. Furthermore, the belt itself becomes more flexible which in turn improves the image quality, however, the strength of the metal heating layer decreases.
Further, the fixing member fuses toner unfixed toner images on the toner on the recording medium while applying pressure to the toner to firmly fix the toner to the recording medium. For this purpose, a press member (e.g., press roll, press pad, press belt and the like) disposed at a position opposed to the fixing belt is used such that a nip load is applied between the fixing belt and the press member. At this point, if the metal heating layer is thin, in some cases, the nip load necessary for fixing causes defects such as cracks or splits.
Moreover, even when the nip load is low, the heating layer is passed through the nip many times causing repeated bending stress, and defects can occur in the metal heating layer such as cracks or splits.
When such defects such as cracks and splits in the metal heating layer may be formed due to bending stress applied repeatedly thereto when the fixing belt passes through the nip, even if the nip load is low.
In such a fixing member, when defects such as cracks and splits are formed, the resistivity of the metal heating layer increases, or the heating property is deteriorated due to occurrence of an electrically insulation in the metal heating layer. Even if the formed cracks do not become splits, but rather groove-shaped defects, the thickness in those regions becomes locally thin, resulting in abnormal heating in the groove-shaped defects. A releasing layer coated on the surface burns or fuses, which drastically deteriorates the durability of the fixing member part due to the abnormal heating
Thus, as disclosed in JP-A No. 2001-341231, a technology has been proposed in which flexibility was imparted to a substrate to thereby reduce the mechanical stress provided on the metal heating layer by using a polyimide resin as a heat resistant resin which constitutes the substrate, and by controlling the imidization rate of the polyimide resin when the substrate is formed.
However, the mechanical stress received by the metal heating layer due to the stress at the nip, is not fully relaxed by merely imparting the flexibility to the substrate, so that the deterioration of durability of the metal heating layer cannot sufficiently be avoided.
Further, in order to solve the problem of the deterioration of the metal heating layer, a method for disposing a protective layer on an outer periphery of a metal heating layer has been proposed (refer to JP-A No. 2004-70191). However, in this method, there are problems of adhesiveness between the metal heating layer and the protective layer, the heat capacity of the protective layer, and the production costs. Accordingly, in order solve the problem, it is considered that the protective layer is made of metal. However, when the protective layer is made of metal, there is a problem that the bending stress resistance is not sufficient, and this method cannot be used as such.